The prior art describes various image reproduction systems and processes that employ photohardenable materials to produce either positive or negative multicolored images. Assignee's patent, Chu et al., U.S. Pat. No. 3,649,268, describes a positive-working image reproduction process employing a layer of photohardenable material on a transparent removable base support. The process comprises laminating the photohardenable layer to a suitable receptor, imagewise exposing the layer through the support to actinic radiation thereby raising the tack temperature of the exposed areas, stripping the support from the layer, and dusting the surface with a colored material which selectively adheres to the unexposed areas of the layer to develop the image. By repeating the laminating, exposing, stripping, and dusting steps in sequence with different color separation records and corresponding toners, a multicolored image is obtained. This kind of photosensitive element, which produces a positive image of the photographic transparency used, is also referred to as a "positive-type" film.
U.S. Pat. No. 3,060,024, to Burg and Cohen, is directed to a positive-working thermal image transfer process which utilizes a photopolymerizable layer. The photopolymerizable layer is imagewise exposed, heated to the tack temperature of the unexposed or underexposed image areas and dusted with a particulate toner so that toner adheres selectively to the tacky image areas. The imagewise-toned photopolymerizable layer is then brought into contact with a receptor, heated to the transfer temperature, subjected to pressure, and, while still warm, separated, whereby the toner particles and underlying photopolymerizable material are transferred to the receptor, giving a positive or duplicate image of the transparency used. The dusting and heat transfer steps may be repeated to produce multiple copies with successive image degradation.
Negative-working reproduction systems which utilize a phototackifiable layer to produce an image are also known. For example, U.S. Pat. No. 4,243,741 teaches a negatively tonable material which contains a combination of at least one dihydropyridine compound and at least one bisimidazole compound. The production of image is based on the photochemical dissociation of bisimidazoles to form tacky, low molecular weight cleavage products. In order to obtain satisfactory image quality, however, it is necessary to prevent recombination of the cleavage products. This can be done by decreasing the viscosity of the layers through internal or external plasticization, thereby increasing the mobility of the cleavage products in the layer. Such layers have a tendency to stain, however, and diffusion or evaporation of the plasticizers from the layers may result in poor storage stability, variable sensitometric properties, and variability of toning.
A negatively tonable reproduction material utilizing photosensitive diazonium salts which form hygroscopic decomposition products is known from U.S. Pat. No. 4,273,842. The decomposition products become tacky and tonable by absorbing water. To achieve uniform toning, however, exposure and toning must be carried out under conditions of carefully controlled humidity. In addition, these layers have a tendency to excessive tackiness and the diazonium salts must be present in crystalline form at least on the surface of the layer or the layer will be tacky even before exposure. This leads to poor differentiation between exposed and unexposed areas, as well as fogging and spot formation.
Another negatively tonable imaging system is taught by U.S. Pat. No. 4,604,340. The photosensitive element of this system is a 4-(2-nitrophenyl)-1,4-dihydropyridine compound which becomes tacky and tonable on exposure to light. The exposing and toning steps may be repeated several times as desired using different images for exposure and different powders for toning. U.S. Pat. No. 4,698,293, on the other hand, teaches a positively tonable image reproduction system that utilizes a light-sensitive layer containing at least one 1,4-dihydropyridine. The light-sensitive layer is exposed imagewise, treated with a strong acid, post-exposed diffusely to produce tacky areas, and subsequently toned with a particulate toner.
In Assignee's patents, Cohen et al., U.S. Pat. Nos. 4,247,619 and 4,174,216, a negative working multilayer photosensitive tonable element and the process for image reproduction using it are described. This element comprises a cover sheet; a photoadherent layer; a tacky a nonphotosensitive contiguous layer receptive to colorant or particulate material for applications such as color proofing; and, optionally, a sheet support. The image reproduction process comprises (a) imagewise exposing the multilayered photosensitive element to actinic radiation, (b) stripping off the cover sheet from the element with the exposed areas of the photohardenable layer adhered to the cover sheet while the nonexposed areas of said photoadherent layer remain on the contiguous layer, and (c) toning by embedding colorant or particulate material in the areas of the contiguous layer uncovered by removal of the overlying photoadherent layer.
One method of manufacturing the above described multilayer photosensitive element is by first coating the photoadherent layer on the cover sheet, drying, and then solvent coating the contiguous layer on the dried photoadherent layer. After drying this layer the element may be laminated to a support. One problem with this process is that the solvent used for coating the contiguous layer has to be carefully chosen so as to have a deleterious effect on the photoadherent layer. Another method of manufacture involves coating the photoadherent layer on the cover sheet and the contiguous layer on the support. After drying, the photoadherent layer and the contiguous layer are laminated together under pressure, either at room temperature or elevated temperatures. A third method involves extrusion coating the photoadherent layer and contiguous layer simultaneously and in that order on the cover sheet, and then laminating the support to the outer surface of the contiguous layer.
The above described manufacturing processes are expensive since they involve coating of two separate layers. Further, the image reproduction process is subject to operator-induced variability since it relies on altering the balance of the adhesive forces between the layers of a multilayer element by exposure to actinic radiation.
The process of the invention allows an easily manufactured "positive-type" film, comprising a support, a photohardenable layer, and, optionally, a transparent cover sheet, to be used to produce either a positive or a negative image.